The present invention relates to an overvoltage protection circuit of a redundant parallel configuration, and to a power source, power supply system, and electronic apparatus using the overvoltage protection circuit.
A power source may be designed to take a redundant parallel configuration for improved reliability. When connecting multiple power sources in parallel to construct a power supply system of a parallel configuration in which the power sources are to be operated in parallel, more power sources than the N number of units actually required may be connected in parallel and operated in parallel (i.e., the power supply system may be designed into a redundant parallel configuration). Accordingly, even in the event of a power source failure, the power supply system does not stop and continues to operate for its improved reliability. When the improvement of power supply system reliability is achieved, the reliability of an electronic apparatus which incorporates the power supply system will improve and the quality of the service that the electronic apparatus provides will also improve. The redundant parallel configuration of power sources applies in cases such as operating N+1 power sources in parallel or operating a greater number of sources in parallel.
The conventional technologies for realizing the redundant parallel configuration include those which use parallel-use diodes (OR diodes). These technologies are described on, for example, page B-2 of “Comprehensive Catalog of Switching Power Supplies—2002” or pages B-214 to B-218 of “Application Notes on Parallel Operation”, both created by Densei-Lambda K. K. According to these conventional technologies, parallel operation is realized by connecting a first terminal side (for example, an anode terminal) of one of multiple diodes in series to one of the output terminals (for example, high-potential side) of each power source and connecting second terminal sides (for example, cathode terminals) of the multiple diodes in common.
In addition, the technologies for realizing the redundant parallel configuration are already known. For example, as detailed in the UC3902 or UC3907 Data Sheets issued by Texas Instruments Incorporated or Unitrode Corporation or in the Application Notes “UC3907 Load Share IC Simplifies Parallel Power Supply Design” or “The UC3902 Load Share Controller and Its Performance in Distributed Power Systems” issued by the same companies, there exists a parallel operation technology for equalizing a load current between multiple power sources by monitoring the respective load currents and increasing the output voltage of the power source having a smaller load current. This parallel operation technology is used to operate the power sources in parallel, regardless of whether or not a redundant parallel configuration is employed.
FIG. 1 is a functional block diagram of power sources, showing an example of a conventional technology. In this figure, power sources 100 and 200 are connected to take a redundant parallel configuration. In this example of a conventional technology, OR diodes 103 and 203 are connected in series to the output terminals 102 and 202 of both power sources, and the output terminal 300 of the power supply system is formed by connecting the cathode terminals of the OR diodes in common to realize the redundant parallel configuration. The parallel operation technology exists in the internal circuits of the power sources. Inside the power source 100, electric power from an input terminal 101 is output to the output terminal 102 via a power circuit 105. This power circuit functions as a voltage converter. At the same time, the power circuit performs the basic functions of the power source, such as stabilizing its output voltage, since the output voltage is controlled by an error amplifier 106 and since the voltage of a remote sense terminal 104 is controlled so as to equal the voltage sent from a reference voltage source 113. The current to the output terminal 102, i.e., the output current is detected at a resistor 107. The detected output current becomes an output current detection voltage in a current detection circuit 108, is output to a parallel operation control terminal 110 via a resistor 109, and coupled with a parallel operation control terminal 210 of the power source 200. Since both parallel operation control terminals 110 and 210 are connected, the voltage developed across the resistor 109 reflects the difference of the output currents between the power source 100 and the power source 200. This differential current is detected by a current comparison circuit 111, then the corresponding voltage is added to the reference voltage 113 by an adder 112, and the output voltage of the power circuit 105 is controlled. A current balance is thus taken for reduced differential output current. Parallel operation is realized by achieving a current balance between the power sources in this manner.
An overvoltage detection circuit 114 stops the operation of the power source to protect a load or the power source itself if a failure in the power source unusually increases the output voltage. This overvoltage detection circuit is a major constituting element of the overvoltage protection circuit provided within the power source and may include a deactivation circuit that operates when the output voltage decreases, and/or reporting circuits on the state and other factors of the power source, although these circuits are not shown. The overvoltage detection circuit, although connected to the cathode terminal of the OR diode in this circuit diagram, may be connected to the anode terminal instead. Alternatively, although this is not shown, the foregoing remote sense terminal may be omitted.
According to this conventional technology for realizing a redundant parallel configuration, if the output voltage is reduced by a failure in one power source, since the OR diode is reverse-biased, the corresponding power source is isolated from other normal ones to allow continued output of a required output voltage from the power supply system and continued normal operation of a section such as an electronic circuit provided as the load. Even if the failure is due to an event such as short-circuiting of a semiconductor, a capacitor, or the like, and results in short-circuiting across the output of a power source, the OR diode acts to prevent output current short-circuiting of other power sources, allowing the power supply system to continue to output the required output voltage. The fact that a power source has failed can be detected by using a method such as monitoring the output terminal 102 of the power source or an anode voltage of the OR diode 103, and it is also possible to deactivate the power source that has failed, and to report the failure to an external monitoring system.
Reliability of the power supply system of a redundant parallel configuration in the above case is calculated for trial below. The calculation assumes an arrangement of N+1 power sources, wherein N is 1 and two power sources are connected in parallel. The calculation also assumes that reliability “ë” of the individual power sources is expressed as 1,000 fit (failures in time) and that a “mean time to repair” (MTTR) in case of a power failure is 24 hours. Since a “mean time between failures” (MTBF) in a power source whose reliability is 1,000 fit is 1 million hours (approx. 114 years), either of the two power sources fails every 500,000 hours (57 years) on the average and is repaired or replaced with a new (normal) one during a repair time of 24 hours. Since the redundant configuration is disturbed during the repair time, if another power failure occurs during the time, the power supply system will be deactivated. The probability of this occurring, however, is about 1/40,000 (=24/1,000,000), so the power supply system will be deactivated when the occurrence of a power failure and repair is repeated 40,000 times, i.e., once every 500,000 hours×40,000 times=20,000 million hours (approx. 2.3 million years). It follows that an ideal value of the reliability of this power supply system is 0.05 fit (MTBF=20 billion hours). For a power supply system not of a redundant parallel configuration, i.e., for a single power source, reliability is 1,000 fit. Use of such redundant parallel configuration technology as described above allows the reliability of a power supply system to be ideally improved from 1,000 fit to 0.05 fit, and the reliability improvement ratio obtained using the redundant parallel configuration technology is 1,000/0.05=20,000 times as an ideal value.
If N=4, redundant parallel connection of five power sources ideally yields 0.48 fit in terms of reliability, and 1,000 fit×4 units=4,000 fit is yielded in a non-redundant parallel configuration. A reliability improvement ratio of 4,000/0.48=8,333 times can therefore be obtained as an ideal value.
As described above, redundant parallel operation is very greatly effective for the improvement of reliability, provided that redundancy works well.
In addition to the above, a related technology is described in Japanese Patent Laid-Open No. Hei 4-372525.